Wireless networks, in which a user equipment (UE) such as a mobile handset communicates via wireless links to a network of base stations or other wireless access points connected to a telecommunications network, have undergone rapid development through a number of generations of radio access technology. The initial deployment of systems using analogue modulation has been superseded by second generation (2G) digital systems such as GSM (Global System for Mobile communications), typically using GERA (GSM Enhanced Data rates for GSM Evolution Radio Access) radio access technology, and these systems have themselves been replaced by or augmented by third generation (3G) digital systems such as UMTS (Universal Mobile Telecommunications System), using the UTRA (Universal Terrestrial Radio Access) radio access technology. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the proposals by the Third Generation Partnership Project (3GPP) of the Long Term Evolution (LTE) system, using E-UTRA (Evolved UTRA) radio access technology, which offers potentially greater capacity and additional features compared with the previous standards.
Note that the term “GERA” is used herein to refer to the radio access technology associated with GERAN (GERA networks), “UTRA” is used to refer to the radio access technology associated with UTRAN (UTRA networks), and similarly the term “E-UTRA” or “LTE” is used to refer to the radio access technology associated with E-UTRAN (E-UTRA networks).
LTE deployments may not support voice communication in all service areas, as in some service areas these may support data communication alone. LTE is designed primarily as a high speed packet switched network, and voice services, packet switched voice services and in particular Voice over Internet Protocol Multimedia Subsystem (VoIMS) services are envisaged, whereas previous generation systems such as UMTS support voice services that are primarily circuit switched.
As new technology is introduced, networks are typically deployed which include radio access networks that use a radio access technology according to a recent standard and also legacy radio access networks that use a legacy radio access technology, and furthermore there may be features that are not available in certain service areas. It is thus desirable for a user equipment to be capable of communication using two or more radio access technologies, so for example the user equipment is able operate using one radio access technology, perhaps offering high capacity, where this is available, but being able to operate using a legacy radio access technology, in those service areas of the network that do not support the other radio access technology, or that do not support preferred features.